The present invention relates to a method of manufacturing tappet for use in a valve-operating mechanism of a direct-acting type in an internal combustion engine.
In order to increase allowable maximum speed and maximum brake power in an engine, DOHC-type engines have been used. Also, in order to lighten the valve operating system, a tappet (valve lifter) for use in the valve operating system has been made of aluminum alloy, instead of conventional steel, such as in Japanese Patent Laid-Open Pub. No. 1-815607.
Aluminum alloy tappets have strength, rigidity and wear resistance lower than those in steel tappets, so that wear resistant metal has been generally applied to a contact surface between a rotary cam and the axial end of an engine valve.
FIG. 3 illustrates a known tappet in a valve operating mechanism of a direct acting type, in which 1 denotes a cylinder head: and 2 denotes a tappet body which slidably contacts the cylinder head 1 and comprises a cylinder the upper end of which is closed. With a larger diameter recess 2a and a smaller diameter engage bore 2b formed at the upper surface and inner surface respectively in the tappet body 2 are engaged a wear resistant metal circular shim 3 and a smaller diameter tip 4 respectively.
The lower surface of the tip 4 contacts the axial end of an engine valve 8 connected with the cylinder head 1 by a pair of cotters 5 and 5, a spring retainer 6, a valve spring 7, etc. The upper surface of the shim 3 slidably contacts a rotary cam 9 the center of which is disposed on an axis of the engine valve 8.
In the foregoing tappet used in the direct-acting valve operating mechanism, driving force by the rotary cam 9 acts against the tip 4 as high compression load repeatedly. Therefore, when the upper and lower surfaces of the tip 4 are formed to be flat and the corners of the tip 4 engaged within the engage bore 2b are formed as a right angle, stress is concentrated to the corners, thereby causing a crack 10 as shown in FIG. 3.
The outer circumference of the tappet body 2 is worn, thereby causing abnormal operation such as clattering and surging, so that the axial end of the engine valve 8 partially contacts the lower surface of the tip 4, thereby causing one-sided wear at the contact surface.
In order to overcome the foregoing disadvantages, the object of the invetion is to provide a method of manufacturing a durable and reliable tappet and a method of manufacturing it while preventing concentration of stress to a tip engage portion in a tappet body and preventing one-sided wear at a contact surface between a tip and an engine valve.
According to the present invention, there is provided a method of manufacturing a tappet for use in an internal combustion engine, comprising a tappet body closed at the upper end and having an engage bore at a center of an inner upper surface, and a tip which has upper and lower convex surfaces and a rounded upper circumferential corner having a certain curvature, the tip being fitted in the engage bore, the lower surface of the tip contacting an axial end of an engine valve.
According to the present invention, there is provided a method of manufacturing a tappet as above, comprising the steps of forming an engage bore having a flat upper surface at a center of an inner upper surface of a plastically deformable cylindrical tappet which is closed at the upper end; inserting a tip having upper and lower convex surfaces and a rounded upper circumferential corner having a certain curvature into the engage bore; and pressing the tip strongly until the inner upper surface of the engage bore is plastically deformed to form a concave surface corresponding to the upper surface of the tip and to caulk a downward circumferential projection around the engage bore inwardly.
The advantages of the present invention method of manufacturing are as follows:
a) The upper surface of the tip is convex and the upper circumferential corner is rounded to have a certain curvature, thereby avoiding concentration of stress at the tip circumference engaged with the tappet body. No cracks are caused at the corner, thereby increasing durability and reliablity of the tappet.
b) The lower surface of the tip is convex, thereby preventing partial contact of the axial end of the engine valve on the tip. Thus, one-sided wear at each contact surface is prevented.
c) The concave surface of the engage bore in the tappet body is formed by pressing the tip itself strongly, thereby facilitating the manufacturing process and increasing productivity of the tappet.